The present invention relates to a wireless communication system, a wireless communication method, and a base station device, and particularly to a wireless communication system, a wireless communication method, and a base station device for controlling transmission power or allocating channels in an antenna-rich base station of a cellular wireless communication system.
1. Cellular Wireless Communication System
In mobile wireless communications, a cellular communication system is common because communications are performed in service areas expanding as planes. In the cellular communication system, plural base stations are scattered in service areas, and areas (where terminals can communicate) covered by the respective base stations are connected to each other, so that planar coverage areas can be realized. A configuration of a cellular wireless communication system is shown in FIG. 1. As shown in FIG. 1, plural base stations 1 and plural terminals 10 exist in the system. Terminals 10-1, 10-2, 10-3, and 10-4 wirelessly communicate with abase station 1-1. Each of the base stations 1 is connected to a network device 20 to secure wired communication channels. In FIG. 1, the terminal 10-1 communicates with the closest base station 1-1 from which preferable signals can be received.
Each of the base stations 1 transmits a reference signal (or preamble signal) that is a unique recognition signal to allow terminals to recognize the base station. The reference signal is designed so as to be unique in the corresponding area in terms of a group of signals to be transmitted, a transmission time or frequency, or a combination of a group of signals and a transmission time or frequency. Each terminal 10 receives the unique reference signal transmitted from each base station 1, and measures the reception intensities thereof to be compared to each other, so that each terminal 10 recognizes wireless conditions with adjacent plural base stations 1. Each terminal 10 determines a base station 1 whose reference signal is the highest in the reception intensity as the closest base station. If it is determined that the currently-connected base station that is the highest in the reception intensity (namely, that provides the most preferable reception condition) has been changed to an adjacent one, a handover is conducted to switch the connection to the adjacent base station by which a more preferable reception condition can be expected.
FIG. 1 shows a downlink signal (communications from a base station to a terminal) A and an uplink signal (communications from a terminal to a base station) B for the base station 1-1. The base station 1-1 transmits the downlink signal A and the base station 1-2 transmits a downlink signal C. The base stations simultaneously transmit the signals at the same frequency and times, and thus there is a possibility that the downlink signals A and C interfere with each other.
The terminal 10-1 located at a boundary of a cell receives a desired signal A from the base station 1-1, but simultaneously receives an interference wave C from the base station 1-2. Thus, the terminal 10-1 is affected by the interference wave C. A ratio of interference and noise power to desired signal power is called Signal Interference and Noise Power Ratio (SINR), and is calculated by desired signal power/(interference power+noise power). At a boundary of a cell, interference from another cell is intensified, and the denominator becomes greater. Thus, the SINR is deteriorated and it is difficult to transmit information with a high throughput rate.
2. Fourth Generation Mobile Wireless Communication System
Recently, the technology of a fourth generation mobile wireless communication system (IMT-Advanced) has been actively developed. As IMT-Advanced, there are LTE-Advanced and IEEE802.16m discussed by the standardization organization 3rd Generation Partnership Project (3GPP) and IEEE, respectively. In these communication systems, broadband transmission using frequency bands higher than those employed in conventional communication systems is realized, and an Orthogonal Frequency Division Multiplexing Access (OFDMA) system is applied, so that high frequency usage efficiency can be realized by sharing plural sub-carriers among plural users.
In addition, it has been discussed that a base station and a terminal are provided with up to eight and four Multi-Input Multi-Output (MIMO) antennas, respectively. In such an antenna-rich system, plural antennas and frequency bands are effectively used, so that the link budget may be improved.
In LTE-Advanced and IEEE802.16m, signals are transmitted using two antennas as standard and basic operations even if the base station is provided with four antennas. The base station transmits signals using four antennas in accordance with capabilities of users.
“3GPP TR 36.912, Section 7.1, Downlink spatial multiplexing” is the standard of LTE-Advanced. Further, “IEEE 802.16m D11, Section 16.3.6.1, Downlink MIMO architecture and data processing” is the standard of IEEE802.16m.
3. Related Technology
FIG. 2 is a configuration diagram for showing a baseband transmission signal processor. FIG. 2 shows a block diagram of an MIMO-OFDMA baseband transmission signal processor 100 employed in a wireless communication system such as Long Term Evolution (LTE) discussed by the standardization organization 3GPP or IEEE802.16m discussed by IEEE. The base station device communicates with plural terminals (user i to user k), and generates signals for plural users. In the first place, channel encoders 101 perform Forward Error Correction (FEC) encoding for input transmission data of plural users i to k so as to protect against errors of propagation channels. Next, modulators 102 convert the error correcting-encoded data into modulated signals. The modulated signal is a signal having a constellation on an IQ signal plane such as QPSK, 16 QAM, and 64 QAM. The converted and generated modulated-signals are input to MIMO encoders 103. The MIMO encoders 103 distribute the sequentially-aligned modulated signals to plural antennas.
Outputs of the MIMO encoders 103 are input to power controllers 104. The power controllers 104 adjust the transmission power of each user in accordance with power allocation determined by a scheduler (not shown).
Signals with the power controlled by the power controllers 104 are input to a resource unit mapper 105 in which the signal of each user is mapped to a resource allocated to each user in accordance with frequency resource allocation determined by the scheduler. Mapping to the resource is performed for each antenna. An Inverse FFT (IFFT) 106 converts frequency domain information of each antenna into a time domain signal. A Cyclic Prefix Inserter (CPI) 107 adds a CP to the obtained time domain signal to complete the baseband transmission signal process.
FIG. 3 is a configuration diagram for showing details of the MIMO encoder 103. FIG. 3 shows a case in which up to four antennas are used. A layer mapper 110 sequentially distributes the input modulated-signals to plural layers. In the case of transmitting the signals with four antennas, the layer mapper 110 distributes the signals to all of four layers (four antenna ports). In the case of transmitting the signals with two antennas, the layer mapper 110 distributes the signals to two predetermined antenna ports (two layers corresponding to the antenna ports 0 and 1 in the example of the drawing). Outputs of the layer mapper 110 are input to a pre-coder 111.
The pre-coder 111 performs a process of adding specified weight to the input signals. The specified weight is composed of predetermined complex numbers, and individual weight is added to each layer distributed by the layer mapper. Plural options are prepared as specified weight. The option of weight prepared in advance is called a code book. The code book is shared by terminals.
A terminal receives a reference signal (a preamble signal, a mid-amble signal, or a pilot signal) and adds the weight of the code book to the reference signal on a trial basis to calculate the received SINR. Then, appropriate weight for the terminal is determined on the basis of which weight of the code book is optimum as the SINR. When the appropriate weight is determined, the terminal feeds back the identifier thereof to the base station. On the basis of the feedback information of the identifier indicating the appropriate weight received from the terminal, the base station transmits a signal to the corresponding terminal using the appropriate weight corresponding to the identifier. Therefore, in response to the identifier information of the code book from the scheduler, the pre-coder 111 extracts the corresponding weight from the memory and adds the same to information distributed to each layer. Further, predetermined weight may be added between a base station and a specific terminal without using the feedback information.
In the case of transmitting signals using four antennas, the pre-coder 111 outputs the signals using the antenna ports 0 to 3. In the case of transmitting signals using two antennas, the pre-coder 111 outputs the signals using antenna ports 0 and 1. Therefore, if all users transmit signals using two antennas, power is concentrated in the antenna ports 0 and 1. Thus, transmission amplifiers capable of outputting high transmission power are needed for the antennas 0 and 1.
FIG. 4 is a diagram for showing an example of power control in Fractional Frequency Reuse (FFR), as a concrete example of power control. The horizontal axis of the drawing represents a frequency and the vertical axis represents transmission power in each frequency domain. In FFR, a frequency band is divided into plural sub-bands as shown in the drawing, and transmission output and desired signal power as the numerator of SINR are increased in a specific frequency to reduce affects of interference from an adjacent base station. In addition, transmission power is weakened in a specific frequency to reduce interference to an adjacent base station as the denominator of SINR, so that the throughput of a terminal existing at a boundary of a cell connected to the adjacent station is improved. For example, even if transmission power is weakened in a specific frequency and a terminal that communicates using the frequency is located near a base station, affects of interference from the adjacent base station are small. As described above, a frequency (hereinafter, referred to as a sub-band) that is given priority is selectively used between adjacent base stations, so that interference between cells is reduced. In the example of FFR of the drawing, a sub-band 1 in which transmission power is high is used for a boundary of a cell, and sub-bands 2 and 3 in which transmission power is low are used for the center of a cell. In FFR, transmission power can be adjusted on the basis of allocation of transmission power as shown in the drawing so as to lower the transmission power in the all sub-bands by a certain value or to further suppress the transmission power in a sub-band in which the transmission power is low.
In the FFR technology, transmission power is decentralized in the sub-bands and a limit is put on power of total frequencies, so that transmission power in each sub-band can have a degree of freedom while putting a limit on the total transmission power. However, signals have been transmitted from all antennas at the same power in view of transmission power of each antenna by focusing on each sub-band. It has been impossible to individually change the transmission power of sub-bands between antennas.
If a base station has four antennas but basically transmits signals using only two of them, power is concentrated in specific two antennas among four. Therefore, transmission amplifiers capable of outputting high transmission power are needed for specific antennas. Specifically, if it is assumed that the total amount of transmission power of four antennas, namely, the maximum transmission output of the entire base station device is 40 W, the maximum output of the transmission amplifier of each antenna is ideally 10 W that is calculated by dividing the maximum output of the entire device by the number of transmission antennas. Accordingly, the maximum output of each amplifier can be kept low, leading to reduction in cost and downsizing of the device. However, power is normally concentrated in specific two antennas as described above. Therefore, at least 20 W-class transmission amplifiers are needed as amplifiers connected to specific two antennas.
In general, the higher a transmission amplifier becomes in maximum transmission output, the larger its size becomes. In order to achieve low cost and downsizing of the base station device, there has been needed a method in which there is no difference of transmission power between antennas and amplifiers each having as low maximum output as possible can be employed, a base station device, or a wireless communication system.